Since infrared rays have a merit that its permeability is high with respect to smoke and fog as compared with visible light, infrared imaging is possible night and day. Furthermore, since temperature information of a subject can also be obtained in the infrared imaging, the infrared imaging has a wide application range as in surveillance cameras and fire detection cameras not to speak of the defense field.
In recent years, development of “uncooled infrared imaging device” which does not need a cooling mechanism has become vigorous. The uncooled, i.e., thermal type infrared imaging device converts incident infrared rays having a wavelength of approximately 10μ to heat by using an infrared absorption film, and converts a temperature change in a heat sensitive part generated by the converted feeble heat to an electric signal by using some thermoelectric conversion element. The uncooled infrared imaging device obtains infrared image information by reading out the electric signal.
For example, an infrared imaging device using a silicon pn junction which converts a temperature change to a voltage change by giving a constant forward current is known (see, for example, JP-A-2002-300475 (KOKAI)). This infrared imaging device has a merit that mass production is possible by using a silicon LSI manufacturing process, using a SOI (Silicon on Insulator) substrate as a semiconductor substrate. Furthermore, since a row selection function is implemented by utilizing rectification characteristics of a silicon pn junction which is a thermoelectric conversion element, the infrared imaging device also has a merit that the pixel structure can be constructed extremely simply.
One of indexes which represent performance of the infrared imaging device is NETD (Noise Equivalent Temperature Difference) which represents the temperature resolution of the infrared imaging device. It is important to make the NETD small, i.e., make the detected temperature difference corresponding to noise small. For making the NETD small, it is necessary to raise the signal sensitivity and reduce the noise.
Threshold voltage clamp processing for reducing the influence of the threshold variation of the amplification transistor is described in JP-A-2002-300475 (KOKAI). In this threshold voltage clamp processing, if a sampling transistor turns on, a negative charge is stored in a gate of an amplification transistor capacitor-coupled to a signal line. At this time, it is desirable to cause a voltage across a coupling capacitance between a signal line and the amplification transistor to converge to (Vdd−Vref)−Vth. Here, Vdd is a bias voltage given to a pixel by a row selection circuit, Vref is a voltage given from a constant current source to a signal line, and Vth is a threshold voltage of a pixel. In the threshold voltage clamp processing, the variation of the threshold voltage in amplification transistors in respective columns can be compensated at the time of signal readout. The instant the threshold voltage is clamped, however, a noise component existing on a signal line is held and thereafter its information is always referred to at the time of the row selection. This results in a problem that longitudinal streak noise appears.